Cooling for X-ray systems

ABSTRACT

A cooling system for an X-ray system with a bearing assembly having a bearing stator and a bearing rotor, includes a cooling stem disposed within the bearing assembly for dissipation of heat from the X-ray system. The cooling stem has dimensions adapted to be disposed within an axial bore of the bearing assembly. The cooling stem consists of a hollow, tubular housing having a target end, a distal end, and a number of radial fins integral with the outer surface of the tubular housing. The radial fins extend longitudinally from the target end in the direction of the distal end to a transition point. The radial fins, in combination with the outer surface of the tubular housing and the inner surface of the axial bore, form a number of axial channels for channeling a cooling medium from the target end to the distal end in a turbulent flow.

BACKGROUND OF THE INVENTION

The instant invention is directed in general to X-ray systems, and morespecifically to improved cooling of the bearing assemblies therein.

An X-ray tube includes a cathode assembly and an anode assembly mountedin an evacuated glass frame or housing. The anode assembly includes atarget in the form of a disk that is rotated at high speed adjacent tothe cathode assembly, which cathode assembly emits an electron beamagainst a focal track adjacent to a perimeter of the target. A smallportion of the electron beam is converted at the focal track into anX-ray beam which passes through a window in the housing for use inimaging or other conventional manners.

In an X-ray tube, less than about 1% of the electrical energy consumedby the tube is converted into X-rays, with the majority of the remainingenergy producing waste heat in the target. Consequently, dissipation ofthe waste heat from the target is critical to the proper functioning ofthe X-ray tube. The X-ray tube is typically immersed in a cooling fluid,such as oil, which is channeled over the outside of the tube forremoving the heat during operation. The heat generated at the targetinside the tube housing, however, must also be dissipated to avoidoverheating.

The X-ray tube is typically operated in cycles having one period inwhich X-rays are generated followed in turn by a cooling period to allowa reduction in temperature of the various components of the tube forpreventing heat damage and eventual failure of the component parts.During the first few minutes of the cooling period, the heat transferfrom the target is predominately radiational, with radiation heattransfer being proportional to the fourth power of temperature. Afterthe initial radiation cooling period, heat transfer is dominated byconduction from the target through the remainder of the anode assemblyto the tube housing.

Since the target rotates during operation, it is mounted on ball orjournal bearing assemblies, the bearing assemblies themselves havingtemperature limits during operation. Conduction of heat from the targetnecessarily heats the supporting bearing assemblies, and ultimatelydecreases the life of this component of the X-ray tube.

In related art inventions, it is known to use a cooling medium flow intoa cylindrical cavity of the bearing assembly to provide cooling for thebearing assembly. Typically, the cooling medium is forced into thecylindrical cavity to the end face of the cavity, and subsequently flowsbask through a cooling duct within the cavity. Such a device isdisclosed within a publication entitled "Leitfaden der medizinischenRotgentechnik" by van der Plaats, 1961. Additionally, a cooling deviceinserted within a cavity to promote cooling is disclosed within Golitzeret al., U.S. Pat. No. 5,094,927. Golitzer discloses a cooling deviceinserted within a bearing cavity to distribute a cooling medium withturbulent flow for improved cooling of the bearing. Golitzer'sdisclosure, however, includes a plurality of discs which extendtransversely to channel the cooling medium back and forth within thecavity to create a flow. This necessary additional movement of thecooling medium creates additional pumping costs within the GolitzerX-ray system.

Therefore, it is apparent from the above that there exists a need in theart for an apparatus for improved cooling of bearing assemblies withinan X-ray tube. In particular, it is desirable for a cooling device toprovide improved cooling effects within a bearing assembly withoutcreating additional pumping costs associated with a complicated, backand forth movement of cooling medium within the device. It is a purposeof this invention, to fulfill this and other needs in the art in amanner more apparent to the skilled artisan once given the followingdisclosure.

SUMMARY OF THE INVENTION

The above-mentioned needs are met by the instant invention whichprovides an X-ray system with a bearing assembly having a bearingstator, a bearing rotor, and a cooling stem positioned within thebearing assembly for increased dissipation of heat from the X-raysystem.

In one embodiment of the instant invention, the cooling stem is disposedwithin an axial bore of the bearing assembly and comprises a hollow,tubular housing having an inner surface, an outer surface, a target end,and a distal end. The cooling stem further comprises a plurality ofradial fins integral with the outer surface of the tubular housingextending longitudinally from the target end in the direction of thedistal end to a transition point. The radial fins, in combination withthe outer surface of the tubular housing and the inner surface of theaxial bore within the bearing assembly, form a number of axial channelsfor channeling a cooling medium from the target end to the distal end ina turbulent flow. Turbulent flow of the cooling medium within the axialchannels provides higher heat transfer from the bearings then a coolingmedium circulation system without turbulent flow.

Additionally, the structure of the instant cooling stem within an X-raysystem, provides sufficient cooling flow that is predominantlylongitudinal along the outer surface of the cooling stem. Therefore, therequired X-ray system cooling flow is generated with less pressure headthen is needed in related art X-ray systems, reducing the X-ray systempumping needs and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding part of thespecification. The invention, however, may be best understood byreference to the following description taken in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a cross-sectional view of a representative X-ray system havingan X-ray tube positioned therein;

FIG. 2 is a plan view of one embodiment of the bearing assembly of theinstant invention;

FIG. 3 is a plan view of one embodiment of the cooling stem of theinstant invention;

FIG. 4 is a target end view of one embodiment of the cooling stem of theinstant invention;

FIG. 5 is a distal end view of one embodiment of the cooling stem of theinstant invention; and

FIG. 6 is plan view of the cooling stem disposed within the bearingassembly as disclosed within the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

An X-ray system 10 comprises an anode assembly 24, a cathode assembly26, and a center section 28 positioned between anode assembly 24 andcathode assembly 26. A representative X-ray system 10 is depicted inFIG. 1. Center section 28 contains an X-ray tube 30. X-ray system 10further comprises a casing 52 typically made of aluminum and lined withlead, a cathode plate 54, and a rotating target 56 enclosed in anenvelope 60, envelope 60 typically made of glass or metal. Target 56typically has a backing plate 58, often made of graphite. Casing 52 isfilled with a cooling medium, often oil, for cooling purposes. A window64 for emitting X-rays is formed in casing 52, relative to target 56 forallowing generated X-rays to exit X-ray system 10.

X-ray tube 30 is operated in alternating periods of X-ray production andcooling to ensure that temperatures of tube 30 and the other X-raysystem components are not overheated. The produced X-rays may be usedfor any conventional purpose.

Referring to FIG. 2, there is shown a bearing assembly 100 including abearing stator 102 and a bearing rotor 104 concentrically surroundingbearing stator 102 defining radially therebetween a journal annulus 106for receiving a suitable lubricant such as liquid Gallium. Target 56 isrotatably supported by bearing assembly 100 which allows rotation oftarget 56 about a centerline axis 108 of X-ray tube 30. Within bearingstator 102 along centerline axis 108 is an axial bore 110 having aninner surface 112, a target end 114, disposed at the end of stator 102closest to target 56, and a distal end 116, disposed at the end ofstator 102 furthest from target 56.

In accordance with the instant invention, a cooling stem 118 comprises ahollow, tubular housing 120 having an inner surface 122, an outersurface 124, a target end 126, and a distal end 128. Cooling stem 118further comprises a plurality of radial fins 130 integral with outersurface 124 extending longitudinally from target end 126 in thedirection of distal end 128 to a transition point 129. Cooling stem 118is depicted in FIGS. 3-5.

In accordance with the instant invention, cooling stem 118 is disposedwithin axial bore 110 of bearing assembly 100, as shown in FIG. 6. Thedimensions of cooling stem 118 are selected such that cooling stem 118is disposed within axial bore 110 so that target end 126 of cooling stem118 is positioned at target end 114 of axial bore 110, leaving a gap 134between cooling stem 118 and target end 114 of axial bore 110 for fluidflew. Distal end 128 of cooling stem 118 is positioned at distal end 116of axial bore 110.

A plurality of axial channels 132 are defined by radial fins 130 incombination with outer surface 124 of tubular housing 120 and innersurface 112 of axial bore 110 within bearing assembly 100. Radial fins130 are sized such that the flow Reynolds number of axial channels 132is within the turbulent region, for example above 2000. Cooling stem 118comprises aluminum or the like. In one embodiment, at least one coolingmedium notch 136 (FIG. 3) is provided at target end 126 of cooling stem118 to allow cooling medium flow from inner surface 122 (FIG. 4) toaxial channels 132.

During operation, heat is conducted from hot target 56 through bearingstator 102, bearing rotor 104, and the lubricant within journal annulus106. The temperature of bearing assembly 100 is highest at the areasclosest to target 56. In order to cool these areas, a cooling medium,often oil, is pumped from a cooling medium supply (not shown) throughdistal end 128 of cooling stem 118, along the path of arrow A in FIG. 6.The cooling medium flows through inner surface 122 (FIG. 4) of coolingstem 118 to target end 114 of axial bore 112 and is forced out of gap134 between cooling stem 118 and target end 114 so that cooling mediumflows from target end 126 towards distal end 128 over outer surface 124of cooling stem 118 through axial channels 132. As mentioned above, thesize of fins 130 is chosen such that the flow Reynolds number of axialchannels 130 is within the turbulent region, for example above 2000.Turbulent flow results in much higher heat transfer coefficients thanlaminar flow. Axial channels 132 create a turbulent flow of the coolingmedium. This turbulent flow increases the heat transfer coefficient attarget end 114 of axial bore 112 and correspondingly enables anincreased dissipation of heat from bearing assembly 100. The heat fromthe hot bearing stator 102, bearing rotor 104, and the lubricant withinjournal annulus 106 at target end 114 of bearing assembly 100 is removedinto the turbulent flow of cooling medium to distal end 128 of coolingstem 118 and out to a cooling source (not shown), connected to coolingstem 118, where the heat is removed from the cooling medium. In thisway, heat is more effectively removed from X-ray bearing assembly 100,the enhanced heat transfer enabling more frequent operation cycles whilemaintaining bearing assembly 100 within necessary temperature limits.

Because heat travels via conduction from hot target 56 to bearing rotor104, the lubricant within journal annulus 106, and bearing stator 102, atemperature gradient is established in the axial direction of bearingrotor 104, the lubricant within annulus 106, and bearing stator 102. Dueto this axial gradient, a higher heat transfer coefficient is needed attarget end 114 of bearing assembly 100 and a relatively lower heattransfer coefficient is sufficient at distal end 116 of bearing assembly100. A high heat transfer coefficient, in general, requires a higherpressure drop to maintain the required cooling medium flow. A higherpressure drop necessitates larger pump capacity or longer pump operationand an overall increase in pumping costs.

Accordingly, in one embodiment of cooling stem 118, radial fins 130 runin a longitudinal direction from target end 126 only to a transitionpoint 129 between target end 126 and distal end 128, and at transitionpoint 129, angle, at some transitioning slope, often between 4° and 11°,to outer surface 124, creating one annular path at distal end 128 ofcooling stem 118. In this embodiment, the higher pressure drop needed tomaintain the high heat transfer coefficient is required only for theflow within axial channels 132, which corresponds to the hightemperature region closest to target 56. The heat transfer needs ofbearing assembly 100 are reduced in the regions further away from hottarget 56, therefore, the high heat transfer coefficient is no longerrequired. Accordingly, at transition point 129, fins 130 slope, at someangle, back to outer surface 124, creating one annular path at distalend 128 cooling stem 118, and lowering the pumping requirements forX-ray system 10. The pumping requirements for this embodiment of coolingstem 118 are thereby significantly lower than would be needed if coolingchannels 132 were to run the entire length of cooling stem 118.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

We claim:
 1. A cooling stem having dimensions adapted to be disposedwithin an axial bore of an X-ray bearing assembly, said axial borehaving an inner surface, said cooling stem comprising:a hollow, tubularhousing having an inner surface, an outer surface, a target end and adistal end; said tubular housing further comprising a plurality ofradial fins integral with said tubular housing outer surface andextending longitudinally from said target end in the direction of saiddistal end to a transition point; and said radial fins disposed alongsaid outer surface such that said fins in combination with said innersurface of said axial bore form a plurality of axial channels forchanneling a cooling medium from said target end to said distal end in aturbulent flow.
 2. A cooling stem, in accordance with claim 1, whereinsaid cooling stem comprises aluminum.
 3. A cooling stem, in accordancewith claim 1, wherein said radial fins, at said transition point, angleat a transitioning slope to said outer surface of said tubular housingcreating one annular path of flow at said distal end of said tubularhousing.
 4. A cooling stem, in accordance with claim 3, wherein saidtransitioning slope is between 4° and 11°.
 5. A cooling stem, inaccordance with claim 1, wherein said radial fins and said outer surfaceof said hollow, tubular housing are sized such that the flow Reynoldsnumber of said axial channels is within the turbulent region.
 6. AnX-ray system having a rotatable target assembly, comprising:a bearingassembly disposed to rotatably support said target assembly, saidbearing assembly comprising a bearing stator, a bearing rotor, and anaxial bore having an inner surface; and a cooling stem having dimensionsadapted to be disposed within said axial bore, said cooling stemcomprising a hollow, tubular housing having an inner surface, an outersurface a target end, a distal end, a plurality of radial fins integralwith said outer surface of said tubular housing extending longitudinallyalong said tubular housing from said target end in the direction of saiddistal end to a transition point, said radial fins in combination withsaid outer surface of said tubular housing and said inner surface ofsaid axial bore forming a plurality of axial channels for channeling acooling medium from said target end to said distal end in a turbulentflow.
 7. An X-ray system, in accordance with claim 6 wherein saidcooling stem comprises aluminum.
 8. An X-ray system, in accordance withclaim 6, wherein said radial fins, at said transition point, angle, at atransitioning slope to said outer surface of said tubular housingcreating one annular path of flow at said distal end of said tubularhousing.
 9. An X-ray system, in accordance with claim 8, wherein saidtransitioning slope is between 4° and 11°.
 10. An X-ray system, inaccordance with claim 6, wherein said radial fins and said outer surfaceof said hollow, tubular housing are sized such that the flow Reynoldsnumber of said axial channels is within the turbulent region.